Notes

Note:
flock() locks mandatory under Windows.

Note:
Because flock() requires a file pointer, you may have
to use a special lock file to protect access to a file that you intend
to truncate by opening it in write mode (with a "w" or "w+" argument to
fopen()).

Warning

flock() will not work on NFS and many other networked
file systems. Check your operating system documentation for more
details.

On some operating systems flock() is implemented at
the process level. When using a multithreaded server API like ISAPI you
may not be able to rely on flock() to protect files
against other PHP scripts running in parallel threads of the same server
instance!

flock() is not supported on antiquated filesystems like
FAT and its derivates and will therefore always
return FALSE under this environments (this is especially true for
Windows 98 users).